The present invention is directed to bicycles and, more particularly, to a hydraulic apparatus for a bicycle brake lever device.
In recent years, some high performance bicycles have been equipped with hydraulic disk brake systems. Hydraulic disk brake systems typically include a caliper housing, first and second brake pads mounted on opposite inner sides of the caliper housing so as to face each other, and one or more pistons that move the first and/or second brake pads towards each other in response to hydraulic pressure created by the actuation of a brake lever assembly mounted to the bicycle handlebar. A rotor that rotates with the bicycle wheel is disposed between the first and second brake pads. The first and second brake pads apply a frictional force to the rotor when they move towards each other in response to the hydraulic pressure created by the actuation of the brake lever assembly, thereby stopping rotation of the rotor and the bicycle wheel.
The brake lever assembly typically comprises a base member structured to attach to the bicycle handlebar and a brake lever pivotably coupled to the base member. The base member includes or is attached to a master cylinder and a fluid reservoir that supplies brake fluid to the master cylinder through appropriate orifices. The fluid reservoir has a removable cap so that brake fluid may be added as necessary. The master cylinder includes a brake fluid outlet and a piston that reciprocates in response to pivoting of the brake lever. A tube is connected between the brake fluid outlet and the caliper housing to communicate brake fluid between the master cylinder and the caliper housing. Pivoting the brake lever toward the handlebar causes the brake lever to press against the piston so that the piston forces brake fluid through the brake fluid outlet to the caliper housing, thereby causing the first and second brake pads to contact the rotor.
Brake fluid is a substantially compressionless fluid that allows the brake fluid to properly communicate the force from the piston in the brake lever assembly to the piston(s) in the caliper housing. Accordingly, it is necessary to ensure that there is no air, which is highly compressible, in the path from the piston in the brake lever assembly to the piston(s) in the caliper. Conventionally, this is accomplished by installing a bleeder screw at the fluid reservoir in the caliper housing. The bleeder screw typically has a solid inner end and an internal fluid passage that extends from a side of the screw near the inner end to the outer end of the screw. When the bleeder screw is screwed into the caliper housing, the solid inner end closes an orifice at the fluid reservoir. Loosening the bleeder screw allows brake fluid to move past the solid inner end, through the internal fluid passage and out of the outer end of the screw. Thus, by loosening the bleeder screw and operating the brake lever, fluid is forced from the reservoir in the brake lever assembly, through the tubing connecting the master cylinder to the caliper housing, and through the bleeder screw. Any air in the path from the master cylinder to the caliper housing exits through the bleeder screw together with any air entrained in the brake fluid. The bleeder screw then is tightened to seal the system. However, sometimes entrained air in the fluid reservoir may enter the master cylinder and be introduced into the path from the piston in the brake lever assembly to the piston(s) in the caliper, thereby degrading braking performance.